A Rhombic Pt(II) Metallacycle with Enhanced Third-Order Nonlinear Optical Response and Optical Power Limiting Performance.

Coordination-driven self-assembly provides an effective strategy for the precise construction of supramolecular nonlinear optical (NLO) materials, yet its potential in third-order NLO applications remains insufficiently explored. Herein, we report a rhombic Pt(II) metallacycle (MPt) constructed via Pt-N coordination-driven self-assembly, achieving markedly enhanced third-order NLO responses that are confirmed to originate from the formation of the metallacyclic structure. Benefiting from the enhanced spin-orbit coupling (SOC) induced by multinuclear Pt(II) nodes and framework rigidification, MPt promotes intersystem crossing (ISC), suppresses triplet-state nonradiative decay, prolongs the triplet-state lifetime to 8.18 μs and strengthens excited-state absorption (ESA). As a result, MPt displays pronounced reverse saturable absorption (RSA) at 532 nm, outperforming the benchmark NLO material C60. Furthermore, the fabricated MPt@PDMS film exhibits excellent optical power limiting (OPL) performance, with an effective nonlinear absorption coefficient (βeff) of 1045.70 cm GW-1 and a low optical limiting threshold (FOL) of 0.23 J cm-2, placing it among the best-performing polymer-based OPL films reported to date. This work provides valuable insight into the rational design of high-performance supramolecular NLO assemblies and establishes supramolecular topological engineering as an effective strategy for amplifying third-order NLO responses.